The present invention relates to turbomachine aerofoils, and is particularly concerned with turbomachine aerofoils which have been produced by casting and hot isostatic pressing processes.
It is well known in the art to produce turbomachine aerofoils, whether a turbomachine blade or a turbomachine vane, by casting molten metal into a mould and allowing the molten metal to cool in the mould and to take the shape defined by the mould. One of the problems associated with the casting process is that there is a possibility of voidage, or porosity, in the finished cast component. It is also well known in the art to remove this unwanted voidage, or porosity, by the use of a hot isostatic pressing process. This involves placing the finished cast component into an autoclave, i.e. a high pressure vessel, sealing the autoclave and applying high pressures and high temperatures on the cast components in the autoclave.
A problem with metals, or alloys, which exhibit a high degree of voidage, or porosity, for example gamma titanium aluminides is that surface irregularities, i.e., dimples, may result on the surface of the cast and hot isostatically pressed component. In the case of turbomachine aerofoils, the surface irregularities may have detrimental effects on the performance of the turbomachine aerofoils in operation.
One way to overcome the problem is to cast the component oversize and then to machine the cast and hot isostatically pressed turbomachine aerofoil to exact and shape. However, this machining operation is very expensive. Another way to overcome the problem is to cast the component to size and shape and to fill in the surface irregularities on the cast and hot isostatically pressed turbomachine aerofoil by for example weld deposition.